To send triggers to SAGA via a USB port, you will need the TMSi Python Interface, a hardware solution and the SAGA trigger cable.
To send triggers to SAGA without the need to have a Printer Port to directly connect the Trigger cable to the PC, converting triggers via a hardware module could be the solution. TMSi offers a conversion module (BBTK USB TTL module) that converts the trigger signals sent through the USB port of the PC to the TRIGGER interface of SAGA.
One of the characteristics of the TRIGGER interface on SAGA is 'Negative Logic'. If the input is unconnected or high, the corresponding trigger value for that bit is logic 0. If the input is low or shorted, the trigger value is logic 1. Therefore, by connecting the trigger cable to SAGA, all inputs are set to 1 and all bits are on. If a trigger is sent, this is stored as a negative peak in the file, as can be seen in the figure below.
Often, during postprocessing, the baseline is required to be lower than the trigger values. To invert the 'Negative Logic', a function called 'mask_type' is created in the TMSi Python Interface. By applying this function to the TRIGGERS channel before starting the recording, the baseline is set to 0.
The hardware conversion module and the SAGA Trigger cable allow for 8 bits to be sent to SAGA. It is possible to send bits 0, 1, 2, 3, 4, 5, 6, 7 and 8 and all possible combinations of these bits via the USB input. TMSi advises to use the TMSi Python Interface to measure with SAGAs TRIGGER port on the Docking Station. An example for sending Triggers to SAGA can be found in 'example_psychopy_erp_experiment_saga.py' in the TMSiPlugins folder of the Python Interface.
It is possible to use the Polybench Quick Recording Application for SAGA, however, please be aware that the bit mask is not applied and the baseline will be high.
Always check the LEDs of the conversion module. During proper initialization, the red LED is turned on for a few seconds. It may happen that the code fails to provide an error, even though no triggers are visible in the signal viewer or in the file. This can be checked by the LEDs.